It is well-known that the power amplification stages of typical radio frequency (RF) broadcast transmitters behave in a nonlinear fashion when operated near peak capacity. One simple solution to this problem is to "back off" the power amplifier and only operate the power amplifier below saturation in its linear region. However, backing off the power amplifier tends to reduce the power conversion efficiency of the power amplifier. Additionally, for a given required transmitter output power, the power amplifier used must be larger (and more expensive) than a power amplifier that can be operated at peak capacity.
Furthermore, although backing off would allow the power output of the power amplifier to behave more linearly, backing off would not alleviate the phase distortion of the power amplifier. For modulation schemes that only depend upon modulation of amplitude (such as AM), phase distortion is of relatively little concern. However, for other types of modulation schemes that rely upon both amplitude and phase modulation, phase distortion is an important concern.
An alternative solution, commonly referred to as predistortion, compensates for the distortion caused by the power amplifier by "predistorting" the signal to be amplified with the "inverse" of the transfer characteristic of the power amplifier. FIG. 1 is a simplified block diagram of an exemplary conventional predistortion subsystem 100 for use in a transmitter. A predistorter 101 is coupled to receive a modulation signal to be amplified by a power amplifier 103 and broadcast through an antenna 105. The predistorter 101 operates on the received modulation signal to predistort the modulation signal with the calculated inverse of the transfer characteristic of the power amplifier 103. Thus, ideally, the "predistortion" and the power amplifier distortion cancel each other out to achieve a linear amplification of the output signal. In this example, the predistortion subsystem 100 includes a trainer 107 to monitor the power amplifier input and output signals to determine the distortion caused by the power amplifier 103, which may change over time. The trainer 107 then provides signals to update the predistorter 101 so that predistorter 101 tracks any changes in the transfer characteristic of the power amplifier 103.
FIG. 2 is a simplified block diagram of the conventional predistorter 101 (FIG. 1). Typical conventional predistortion schemes attempt to model the performance of the power amplifier and calculate the "inverse" of the amplifier transfer characteristic. All of the predistortion schemes known to the inventors of the present invention attempt to model the power amplifier performance as a function of the instantaneous power or magnitude envelope of the input signal to the power amplifier. Accordingly, these conventional predistortion schemes predistort the input signal as a function of the power or magnitude of the signal to be amplified. For example, U.S. Pat. No. 4,291,277 issued to Davis et al. and U.S. Pat. No. 5,049,832 issued to Cavers disclose such a scheme. Thus, in this scheme, the predistorter 101 includes a power calculator 201 for calculating the instantaneous power or magnitude of the received modulation signal. The calculated instantaneous power or magnitude is then used to access a look-up table (LUT) 203 that stores a corresponding complex value for this particular instantaneous power or magnitude. This complex value approximates the local inverse of the transfer characteristic of the power amplifier for this particular instantaneous power or magnitude of the modulation signal. The LUT 203 can be periodically updated by the trainer 107 (FIG. 1) so that the complex values reflect any changes in the transfer characteristic of the power amplifier 103 (FIG. 1). The LUT 203 provides this complex value to a multiplier 205, which multiplies the modulation signal with this complex value to predistort the modulation signal. Thus, when the predistorted modulation signal is subsequently amplified by the power amplifier, the predistortion cancels to some extent the distortion caused by the power amplifier.
Although these conventional predistortion schemes represent an improvement over earlier schemes to reduce power amplifier distortion, the inventors of the present invention have observed that modeling the power amplifier transfer characteristic using only the instantaneous power or magnitude does not completely accurately predict the distortion caused by the power amplifier. Consequently, predistortion schemes based on such models cannot completely correct the distortion caused by the power amplifier.
FIG. 3 is a flow chart illustrative of a conventional process of calculating the complex values that are stored in the LUT 203 (FIG. 2). In a step 301, the trainer 107 (FIG. 1) determines the instantaneous magnitude and phase of the input signal to the power amplifier 103 (FIG. 1) and the instantaneous magnitude and phase of the amplified (and distorted) output signal. The trainer 107 typically stores these values in a LUT (not shown) within the trainer and, thus, can directly model the transfer characteristics of the power amplifier 103. In step 302, these characteristics are stored in a first lookup table (LUT). This first trainer LUT is indexed using the actual amplifier output power or magnitude. Then in a next step 303, the trainer 107 calculates the mathematical "inverse" of the transfer characteristic of the power amplifier. This step is generally computationally intensive, thereby undesirably increasing the complexity of the hardware and software of the trainer 107 and consuming processing time and power. In addition, because the trainer LUT is indexed by actual amplifier output power or magnitude, addressing errors may occur because the amplifier output signal is potentially noisy. Then in a step 305, the trainer stores the calculated inverse in the second LUT 203 of the predistorter 101.
In view of the above shortcomings of conventional predistortion schemes, there is a need for a predistortion system that will compensate for power amplifier distortion more accurately than the conventional predistortion systems that are based on instantaneous envelope power or magnitude. There is also a need for a less complex and more accurate scheme to provide the inverse of the power amplifier transfer characteristic.